Hey Joel
I know that the Blackmagic HDMI – SDI converter will not work from my own experience. Like you said, because of the frame rate. The TH2GO is kind of dumb when it comes to EDIDs downstream of the converter.
Another alternative would be to get a router with DVI Extenders and use the router to make the switch.
I think you will have to get some Matrox MXOs or Convert DVIs to get the TH2GO into the ATEM.
Joel Caron